Fix global tool log rotation isolation

[Widthdom]

Summary

• Split persistent global tool stderr logs into per-process files using PID and process start time suffixes.
• Added a default 50 MiB global tool log size cap with CDIDX_GLOBAL_TOOL_LOG_MAX_BYTES, while preserving --log-max-size-mb.
• Updated tests, docs, and changelog fragments for the new log filename and rotation behavior.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~GlobalToolLogTests|FullyQualifiedName~ProgramRunnerTests.Run_ForcedGlobalToolLogging"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ProgramRunnerTests|FullyQualifiedName~GlobalToolLogTests" (run by Codex adversarial review)
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• git diff --check origin/main..HEAD
• Codex adversarial review: No blocking/actionable issues found.

Documentation And Changelog

• Updated README.md, USER_GUIDE.md, and DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/1619.fixed.md.
• Added changelog.d/unreleased/1625.fixed.md.

Fixes #1619
Fixes #1625

Follow-Up Candidates

• None.